Inorganic foundry binder systems and their uses

ABSTRACT

This invention relates to inorganic no-bake foundry binder systems and their uses. The binder systems comprise as separate components: (A) mono-aluminum phosphate in an aqueous solution containing specified phosphoric acids, and (B) magnesium oxide; and certain specified zinc compounds in either the Component A, Component B or both. The components of the binder system react when they are mixed with a foundry aggregate to prepare foundry mixes which are used to prepare foundry molds and cores. The foundry molds and cores are used to cast metals.

This application is a division of application Ser. No. 08/123,507, fileson Sep. 17, 1993 now U.S. Pat. No. 5,382,289.

TECHNICAL FIELD OF THE INVENTION

This invention relates to inorganic no-bake foundry binder systems andtheir uses. The binder systems comprise as separate components: (A) anacid phosphate in an aqueous solution containing specified phosphoricacids, and (B) magnesium oxide; and a compatible zinc compound which ispart of either Component A, Component B, or both. The components of thebinder system are mixed with a foundry aggregate to prepare foundrymixes which are used to prepare foundry molds and cores. The foundrymolds and cores are used to cast metals.

BACKGROUND OF THE INVENTION

There is considerable interest in developing an inorganic foundry binderwhich has the performance characteristics of commercial organic foundrybinders. Organic foundry binders, particularly those based uponpolyurethane chemistry, have been used in the casting industry forseveral decades in both the no-bake and cold-box processes. This isbecause they produce foundry molds and cores with acceptable tensilestrengths that shakeout of castings with relative ease. The castingsprepared with these foundry molds and cores have a good surface finishwith only minor defects.

Currently, the effects of organic foundry binders on the environment andhealth are under study. Consequently, there is an interest inconsidering alternative binders in case these studies are negative.Inorganic foundry binders are of particular interest because they arenot subject to some of the concerns associated with organic foundrybinders.

Various compositions of inorganic foundry binders are known. See forexample U.S. Pat. No. 3,930,872 which describes an inorganic foundrybinder comprising boronated aluminum phosphate and an oxygen-containingalkaline earth metal in specified amounts. Although these bindersproduce molds and cores that have adequate strength and shakeout easilyfrom metal castings prepared with them, the binders exhibit highviscosity, are not very flowable and are not well suited for use withcontinuous mixers. Furthermore, molds and cores prepared with thesebinders do not exhibit adequate humidity resistance.

As another example of an inorganic foundry binder, see U.S. Pat. No.4,111,705 which describes an inorganic no-bake foundry binder comprisingorthophosphoric acid, a ferrous oxide containing material, and awater-soluble alkali metal or ammonium salt of certain carboxylic acids.U.S. Pat. No. 4,430,441, also describes a no-bake inorganic foundrybinder comprising from 95-99 weight percent of a refractory fillercontaining magnesium oxides, iron oxides, silicon oxides or mixturesthereof and from 1 to 5 weight percent of an organic acid having aspecified dissociation constant.

The binders disclosed in the latter two patents are not practical to useon a commercial scale. They do not produce foundry molds and cores withadequate strengths that easily shakeout of the castings prepared withthem, and the castings produced are not substantially free of majordefects. Furthermore, the work time and the strip time of the molds andcores produced with these binders are not acceptable from a commercialperspective.

SUMMARY OF THE INVENTION

This invention relates to inorganic foundry binder systems whichcomprise as separate components:

A. an acid phosphate in an aqueous acid solution of a phosphoric acidselected from the group consisting of orthophosphoric acid,pyrophosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid,polyphosphoric acid, and mixtures thereof; and

B. magnesium oxide; and

a compatible zinc compound which is part of either Component A,Component B, or both.

Preferably, the acid phosphate is mono-aluminum phosphate. The zinccompound is preferably mono-zinc phosphate if the zinc compound is usedin Component A while zinc oxide is preferred if the zinc compound isused in Component B. If the mono-zinc phosphate is used in Component A,it can also act as the acid phosphate.

Preferably, the phosphoric acid is orthophosphoric acid and preferablythe magnesium oxide is a refractory form of magnesium oxide, mostpreferably dead-burned magnesite. If the zinc compound is zinc oxide,and is part of Component B, then the magnesium oxide and zinc oxide arepreferably heat treated as a powder blend.

The invention also relates to foundry binders prepared by mixing andreacting the separate components of the binder system, typically in thepresence of a foundry aggregate to prepare foundry mixes. The foundrymixes are used to form foundry shapes, such as molds and cores, by ano-bake process. The foundry shapes are used to make metal castings.

The molds and cores prepared with these foundry binder systems have goodsurface characteristics with few casting defects. Additionally, themolds and cores readily shakeout of castings prepared with them. Thesand mixture is flowable and can be compacted into cores and molds.

The work time of the foundry mix is generally less than 15 minutes,typically from 6-10 minutes. The strip time is less than 30 minutes,typically from 12-20 minutes. Foundry shapes made with the binder systemtypically exhibit 1 hour transverse disk strengths of 100-200 psi and 24hour transverse strengths of 300-400 psi.

The foundry mixes and foundry shapes also exhibit improved humidityresistance compared to typical systems. Initial casting results indicategood hot strengths with little penetration, veining to a small extent,and good shakeout. Furthermore, the use of these binder systems is notlikely to have a negative impact on human health and the environment.

BEST MODE AND ENABLING DISCLOSURE

For purposes of this disclosure, a foundry binder system comprises theseparate components of the foundry binder, namely Component A andComponent B. The foundry binder is the reaction product which resultsfrom mixing Component A with Component B, typically in the presence of afoundry aggregate. The foundry mix is the mixture of the foundry binderand aggregate, usually sand. Foundry shapes are typically made byshaping the foundry mix into a mold or core. Although the foundry binderis predominately inorganic, it may be desirable to add certain organicmaterials to the binder system as well as other additives.

Component A of the foundry binder system is a liquid which comprises anacid phosphate in an aqueous solution of a phosphoric acid selected fromthe group consisting of orthophosphoric acid, pyrophosphoric acid,trimetaphosphoric acid, tetrametaphosphoric acid, polyphosphoric acid,and mixtures thereof. The concentration of the phosphoric acid in theaqueous solution is typically from 30 to 50 weight percent of thesolution, preferably 35 to 45 weight percent. The concentration of thephosphoric acid solution in Component A typically ranges from 40 to 75weight percent based upon the total weight of Component A, preferablyfrom 50 to 60 weight percent.

An acid phosphate is a mono-substituted phosphate. Examples of acidphosphates use in Component A include mono-aluminum phosphate, mono-zincphosphate, mono-ammonium phosphate, mono-magnesium phosphate,mono-calcium phosphate, and mono-sodium phosphate. Mono-aluminumphosphate is preferably used as the acid phosphate in Component A.

The amount of acid phosphate used in Component A will depend uponwhether there is any mono-zinc phosphate in Component A, whether thereis any zinc oxide in Component B, and how much of these zinc compoundsare present. Typically the amount of acid phosphate, preferablymono-aluminum phosphate in Component A, is from 25 to 55 weight percentbased upon the total weight of Component A, preferably from 40 to 50weight percent.

It is believed that the acid phosphate can be added to the phosphoricacid directly. However, it is preferable to form the acid phosphate inthe phosphoric acid solution by adding an acid phosphate precursor (forinstance by adding aluminum or aluminum oxide). The amount of acidphosphate precursor added to the phosphoric acid is determinedstoichiometrically to provide the desired weight percent ofmono-aluminum phosphate from 25 and 55 weight percent in phosphoric acidsolution.

The addition of the mono-aluminum phosphate precursor can be made atroom temperatures, but the solution of precursor and phosphoric acidmust be heated to produce the desired chemical reaction formingmono-aluminum phosphate. Temperatures between 50° C. and 120° C. arerequired to produce mono-aluminum phosphate.

If mono-zinc phosphate is used alone or in addition to another acidphosphate in Component A, it is believed that the mono-zinc phosphatecan also be added to the phosphoric acid directly. However, it ispreferable to form the mono-zinc phosphate in the phosphoric solution byadding a zinc compound to the phosphoric acid solution which does notresult in the formation of a by-product which remains in the acidphosphate/phosphoric acid solution. Examples of such compounds includezinc phosphate, zinc carbonate, zinc hydroxide, preferably zinc or zincoxide, and most preferably zinc oxide. The addition of mono-zincphosphate precursor should be made at temperatures between 90° C. and110° C., preferably 100° C. When both mono-zinc phosphate andmono-aluminum phosphate are formed by precursor addition to phosphoricacid, the mono-zinc phosphate precursor should be added to the acidfirst at temperatures between 90° C. and 110° C., preferably 100° C.,and allowed to react. Following the mono-zinc phosphate formation, thesolution should be cooled to temperatures between 25° C. and 70° C.before addition of the mono-aluminum phosphate precursor. The additionof the mono-zinc phosphate precursor should be made at temperatures ofabout 90° C. to 110° C., preferably 100° C.

Stoichiometric amount refers to the molar ratio of A or Zn in theprecursor to phosphoric acid. For instance, in the case of aluminumprecursors, one mole of A metal reacts with 3 moles of phosphoric acidto form one mole of Al(H₂ PO₄)₃ (mono-aluminum phosphate) plus 1.5 molesH₂ gas. If Al₂ O₃ (aluminum oxide) is used, 1 mole of Al₂ O₃ reacts with6 moles of phosphoric acid to give 2 moles of Al (H₂ PO₄)₃ plus 3 molesH₂ O.

In the case of zinc precursors, one mole of zinc metal reacts with 2moles of phosphoric acid to give 1 mole Zn(H₂ PO₄)₂ (mono-zincphosphate) plus 1 mole H₂ gas. If ZnO is used as the mono-zinc phosphateprecursor, 1 mole of ZnO reacts with 2 moles of phosphoric acid to give1 mole Zn(H₂ PO₄)₂ plus 1 mole of H₂ O.

The viscosity of Component A is preferably under 400 cps at 25° C., mostpreferably from 100 to 300 cps.

If Component A does not contain any mono-zinc phosphate, then ComponentB must be comprised of a mixture of (1) magnesium oxide and (2) mostpreferably zinc oxide, or presumably any other zinc compound whichthermally decomposes upon heating to give zinc oxide such as, forexample, zinc acetate or zinc carbonate. It should be noted that themixture of zinc oxide and magnesium oxide can also be used as ComponentB even when Component A contains up to about 12 weight percent mono-zincphosphate.

The magnesium oxide used in the Component B is preferably a refractoryform of magnesium oxide, such as dead-burned periclase, most preferablydead-burned magnesite. The weight ratio of magnesium oxide to zinc oxidein the Component B is from 1:9 to 9:1, preferably from 9:1 to 6:4. Themagnesium oxide and the blends of magnesium oxide and zinc oxide aretreated by heating the magnesium oxide or mixtures of magnesium oxideand zinc oxide at temperatures between 1050° C. and 1500° C., preferably1250° C. and 1350° C. to reduce surface area and thereby reducereactivity with the liquid component.

The amount of zinc used in the binder will depend upon whether the zinccompound is used in Component A, Component B, or both, and the weightratio of Component A to Component B. Typically the amount of zinc (whichis usually in the form of a zinc acid phosphate if present in theComponent A or zinc oxide if present in the Component B) in the bindersystem, where the weight ratio of Component A to Component B is 1:1, isfrom 0.5 to 25 weight percent, preferably 1.0 to 20.0 weight percent,and most preferably 1.5 to 10.0 weight percent, said weight percentbeing based upon the total weight of Component A and Component B.

The weight ratio of the Component A to the Component B is generally from3:1 to 1:3, preferably from 3:2 to 2:3. Generally, an effective bindingamount of binder system is such that the weight ratio of foundry bindersystem (which includes Component A and Component B) to aggregate is from1:100 to 10:100, preferably 2:100 to 8:100.

The combination of the binder components provides a binder which hasappropriate work time, strip time, immediate and long term transversestrengths, effective shakeout, and castings free of defects. It alsogives the user significant flexibility to customize a binder appropriatefor his needs which is not likely to have a negative impact on humanhealth and the environment.

Foundry mixes are prepared from the foundry systems by mixing thefoundry binder system with a foundry aggregate in an effective bindingamount. Either the Component A or Component B can be first mixed withthe aggregate. It is preferred to mix the Component A of the foundrybinder system with the foundry aggregate before adding the Component B.

Although the subject binders are preferably and predominately inorganic,optional components, including organic additives, may be added to thebinder, such as polyvinyl alcohol, polyacrylic acid, polyacrylamide,urea, cellulose, citric acid, rubber lattices, and Nylon; and inorganicadditives such as potassium phosphate, chromite and cement. Thoseskilled in the art of formulating inorganic foundry binders will knowwhat substances to select for various properties and they will know howmuch to use of these substances and whether they are best incorporatedinto the Component A, Component B, or mixed with the aggregate as aseparate component. Obviously, it may be necessary to adjust the ratiosset forth previously related to the amounts of components if optionalmaterials are added to the binder formulation.

EXAMPLES

The examples will illustrate specific embodiments of the invention.These examples along with the written description will enable oneskilled in the art to make and use the invention. It is contemplatedthat many equivalent embodiments of the invention will be operablebesides these specifically disclosed.

In the examples, the foundry molds are prepared by the no-bake processmade by mixing Wedron 540 sand with the components of the binder system.Component A of the binder system used in the examples consisted ofmono-aluminum phosphate (MAP) [in some cases mixtures of MAP withmono-zinc phosphate (MZP)] in an aqueous solution of orthophosphoricacid (OA) at 40% concentration. Component B consisted of a mixture ofdead-burned magnesium oxide (MO), and some cases mixtures of MO withzinc oxide (ZO). The MO and the mixtures of MO and ZO were treated at1350° C.

The Component A and sand were first mixed in a Hobart stainless steelmixer for several minutes until thoroughly mixed. Then the Component Bwas added to the sand/Component A mixture and mixed for one minute untilboth Component A and Component B were mixed thoroughly with the sand.The work time (WT) and strip time (ST) for the foundry mixes are givenin Table I.

Transverse strengths of cores made with the binders are also shown inTable I. Measuring the transverse strength of the test samples enablesone to predict how the mixture of aggregate and binder will work inactual foundry operations. In order to measure transverse strengths,transverse disk (50 mm diameter×12 mm height) strength samples wereformed by hand ramming the sand binder mixture into two 12-specimen coreboxes. Then the transverse strengths were measured using a+GF+UniversalStrength Machine Model PFG 1 hour, 3 hours and 24 hours after curing atambient conditions. The humidity resistance was tested by placingsamples which had cured for 24 hours into a humidity chamber at 25° C.and 99% relative humidity (RH).

The abbreviations used in the examples are the following:

OA=orthophosphoric acid

MAP=mono-aluminum phosphate

MZP=mono-zinc phosphate

MO=magnesium oxide

ZO=zinc oxide

Examples 1-3

Examples 1-3 illustrate the effect of placing the zinc compound ineither the Component A, Component B, or both components. A mixturecontaining Wedron 540 silica sand, 3.5% B.O.S. of liquid Component A,and 3.5% B.O.S. of powder Component B, was prepared as previouslydescribed.

In Example 1, the zinc compound was in the Component A as MZP. ComponentA contained 49.2% weight percent OA at 40% concentration weight, 39.7weight percent MAP, and 11.1% MZP, all weight percents based upon thetotal weight of the. Component A. The Component B contained MO heated at1350° C. for three hours, but did not contain a zinc compound. The MOwas ground using a mortar and pestle and sieved to less that 170 mesh toreduce particle size.

The Component A liquid was first added to the Wedron 540 sand in astainless steel mixing bowl and mixed for a total of 1.5 minutes using apaddle style mixer. The powdered Component B hardener was added secondand mixed for a total of one minute.

In Example 2, there-was no zinc compound used in the Component A.Instead the zinc compound (ZO) was used in the Component B with MO as aheat treated blend prepared by heating the ZO and MO at 1350° C. forthree hours. The weight ratio of ZO to MO in the blend was 10:90. Theamount of OA (at 40 percent concentration) used in Component A was 56weight percent and the amount MAP used was 44 weight percent.

In Example 3, there was a zinc compound in both the Component A andComponent B. The Component A of Example 3 was the same as in Example 1and the Component B was the same as in Example 2. The Component Bcontained ZO and MO in a weight ratio of 10:90.

The results of Examples 1-3 are summarized in Table I. Each Component Ain the Examples contained orthophosphoric acid (OA) which is not listedin Table I. The following abbreviations are used in the Table:

EX=Example

CMP=Component

RH=Relative humidity

ST=Strip time

WT=Work time

                  TABLE I                                                         ______________________________________                                                         TRANSVERSE                                                                    STRENGTH (PSI)                                                     CMP     CMP            1    3    24   24 + 1                            EX #  A       B       WT/ST  HR   HR   HR   99% RH                            ______________________________________                                        1     MAP     MO      9.5/21 116  352  350  121                                     MZP                                                                     2     MAP     MO      7/16   234  338  323  110                                             ZO                                                              3     MAP     MO      7/17   103  393  362  66                                      MZP     ZO                                                              ______________________________________                                    

The data in Table I indicate that it is preferred to use the zinccompound in Component A or Component B rather than both.

We claim:
 1. A no-bake process for preparing a foundry mold or corecomprising:(1) mixing at about ambient temperature a foundry aggregatewith an effective bonding amount of up to about 10% by weight, basedupon the weight of the aggregate, of a binder composition comprising asseparate components:A. an acid phosphate in an aqueous acid solution ofa phosphoric add selected from the group consisting of orthophosphoricadd, pyrophosphoric acid, trimetaphosphoric acid, tetrametaphosphoricacid, polyphosphoric acid, and mixtures thereof, and B. magnesium oxide;andwherein a zinc compound in is added to either component A, componentB, or both, and components A and B are added sequentially to theaggregate without regard to the order of addition; (2) introducing thefoundry mix obtained from step (1) into a pattern; (3) allowing thefoundry mix to harden in the pattern until it becomes self supporting;and (4) thereafter removing the shaped foundry mix of step from thepattern and allowing it to further cure, thereby obtaining a hard,solid, cured mold.
 2. The no-bake process of claim 1 wherein themagnesium oxide is dead-burned magnesium oxide and the acid phosphate ismono-aluminum phosphate.
 3. The no-bake process of claim 2 wherein thephosphoric acid of the Component A is orthophosphoric acid.
 4. Theno-bake process of claim 3 wherein the aqueous acid solution oforthophosphoric acid is from 30 weight percent to 50 weight percent oforthophosphoric acid.
 5. The no-bake process of claim 4 wherein theamount of mono-aluminum phosphate is from 25 to 55 weight percent basedupon the weight of Component A.
 6. The no-bake process of claim 5wherein the amount of zinc compound in the binder composition is from1.5 to 10 weight percent, said weight percent being based upon the totalweight of Component A and Component B.
 7. The no-bake process of claim 6wherein zinc oxide is used in Component B as the zinc compound as a heattreated blend with the magnesium oxide, whereby said blend is heattreated at a temperature of from 1250° C. to 1350° C., and whereby theweight ratio of zinc oxide to magnesium oxide in Component B is from 9:1to 1:9.